Here’s a great article from Hatches Magazine (free) on choosing the right fly.
The Big Fly Fishing Debate
If you ask me, there are really two schools of thought in fly fishing. There’s the “presentation” crowd, which centers around a few patterns and holds that you don’t need a fly box full to catch fish. The way the fly is presented to the fish is most important.
The second school is the “match the hatch” crowd. These folk think you need a fly box chock full of flies to cover any and every fishing situation you might run into.
I tend to be a “both” thinker. I have a few flies that are my “go to” flies. I fish these as attractors, trying to find the fish. Once I’ve gotten a few strikes or caught a few fish, I begin to look at what’s in the air and water. If I see rises, I go dry. If I see a caddis hatch, I have a couple of caddis patterns in different shades. If I don’t see any rises, I go nymph. Often, I fish fly combinations, allowing me to test dry flies or floating terrestrials (grasshoppers, etc.).
The article is one place to start.